An electrocardiogram, often called an ECG or EKG, is a standard, non-invasive test that records the heart’s electrical activity. It is used by healthcare professionals to investigate symptoms related to heart function, detect irregular heart rhythms, and diagnose conditions like a heart attack. The procedure is painless and involves placing small electrodes on the skin of the chest, arms, and legs. These electrodes detect the electrical impulses generated by the heart, which are then translated into a visual graph.
The Visual Language of an ECG
An ECG tracing is a line graph plotted on a specific type of grid paper, which creates a visual representation of the heart’s electrical signals over time. The horizontal axis of this grid measures time. The paper is marked with small and large squares, where each small square represents 0.04 seconds and each larger, thicker-lined square represents 0.20 seconds.
The vertical axis of the grid measures the electrical voltage, or amplitude, of the impulses. The vertical grid is marked with small squares, each representing 0.1 millivolts (mV), and the height or depth of the waves indicates the signal’s strength.
Decoding the Waves: P, QRS, and T
The distinct waves on an ECG tracing are labeled with letters and correspond to specific events in the cardiac cycle. The first wave, the P wave, represents atrial depolarization, the electrical activation of the heart’s two upper chambers, the atria. A normal P wave is a small, rounded, and upright deflection lasting less than 0.12 seconds.
Following the P wave is the QRS complex, a larger formation representing ventricular depolarization, the electrical activation of the heart’s powerful lower chambers, the ventricles. The complex consists of the Q wave, the R wave, and the S wave, with a duration between 0.06 and 0.10 seconds.
After the QRS complex, the T wave appears, signifying ventricular repolarization. This is the recovery phase where the ventricles electrically reset before the next heartbeat. The T wave is broader and more rounded than the QRS complex and should point in the same direction as the main deflection of the QRS complex.
Important Intervals and Segments
Beyond the individual waves, the distances and sections between them, known as intervals and segments, provide information about the timing of the heart’s electrical conduction. The PR interval is measured from the beginning of the P wave to the start of the QRS complex. It represents the time it takes for the electrical signal to travel from the atria, through the AV node, and into the ventricles. A PR interval ranges from 0.12 to 0.20 seconds.
The ST segment is the flat line that connects the end of the QRS complex to the beginning of the T wave. This segment corresponds to the period when the ventricles are contracting. Its shape and position relative to the baseline are significant; it should be flat, and any elevation or depression can indicate issues with blood flow.
The QT interval spans from the start of the QRS complex to the end of the T wave. This interval represents the total time required for the ventricles to depolarize and then fully repolarize. The RR interval, the time between two consecutive R waves, is a measurement used to calculate the heart rate.
Understanding ECG Leads and Printout Information
The complete picture of the heart’s electrical activity is captured using multiple leads. Electrodes placed on the skin create these leads, each providing a different electrical view of the heart from a unique angle. A standard diagnostic ECG uses 12 leads, which is why the same wave can appear different across the various tracings on a single report.
Modern ECG machines often provide an automated analysis printed directly on the report. This printout includes calculated values such as the heart rate and the durations of the PR interval, QRS complex, and QT interval. The machine may also offer a preliminary interpretation of the heart’s electrical axis, which describes the overall direction of electrical flow.